verted to carotenoids. Stanier (HI) has reported evidence 

 indicating that the initial compound in this series is phytoene 

 or tetrahydrophytoene (see Figure 10). 



Present evidence indicates that conversion of the C40 

 compound formed from the condensation, to carotenoids, 

 involves a number of dehydrogenations, and finally ring 

 closure at the ends of the molecule. The various oxygen-con- 

 taining carotenoid compounds are probably formed by oxida- 

 tions, hydrations, etc. The structures of a great many of these 

 compounds, both intermediates and end products, have been 

 established in the laboratories of Karrer (112), Zechmeister 

 (113), Inhoffen (114), Weedon (115), and others. 



Chlorophyll and heme 



The pathways to porphyrin compounds have been re- 

 cently reviewed by Granick (116,117), Shemin (118), Rim- 

 ington (119), and Bogorad (120). Some of the key steps from 

 these paths are shown in Figure 11. Glycine and succinate 

 formed from the carbon reduction cycle are the starting com- 

 pounds for the syntheses of these pigments. Glycine may be 

 formed from serine, which in turn is probably synthesized 

 from 2-phosphoglycerate, formed from the 3-phosphoglycerate 

 of the cycle (see the section on Amino Acids). Alternatively, 

 glyoxylate may be transaminated to give glycine. The deriva- 

 tion of this glyoxylate from the carbon reduction cycle is not 

 known for certain, but is probably related to the formation 

 of glycolic acid (see the section on Carboxylic Acids). Thus 

 glycolate formed by oxidation of the glycolyl fragment from 

 the sugar phosphate transketolase system could be further 

 oxidized to glyoxylic acid. A hypothetical split of malate 

 could lead to acetate and glyoxylate. 



If the chloroplast contained isocitritase, both succinate 

 and glyoxylate could be formed by the same reaction on iso- 

 citrate. The isocitrate would in this case come from acetyl 

 CoA and oxalacetate condensation, via citrate. Oxalacetate 



62 



